Ultrasonic diagnosis apparatuses are highly safe medical diagnosis instruments noninvasive to a human body and are smaller in scale than the other medical diagnostic imaging apparatuses such as X-ray diagnosis apparatuses and MRI (Magnetic Resonance Imaging) apparatuses. Further, when a simple procedure is performed to apply an ultrasound probe to a body surface, the ultrasonic diagnosis apparatuses are able to display a real-time image showing the motion of a test object, such as the pulsatory motion of a heart and the movements of a fetus. Consequently, the ultrasonic diagnosis apparatuses are now playing an important role in present-day medicine.
The ultrasonic diagnosis apparatuses transmit ultrasonic waves into a test object when high-voltage drive signals are respectively supplied to multiple transducers built in the ultrasound probe. The multiple transducers respectively receive reflections of the ultrasonic waves, which are generated in accordance with acoustic impedance difference between body tissues in the test object. Based on the reflections of the ultrasonic waves, which are received by the ultrasound probe, the ultrasonic diagnosis apparatuses generate an image.
A transmitter circuit that transmits the high-voltage drive signals to the transducers built in the ultrasound probe is formed of a high-voltage device and capable of generating a high-voltage signal having a peak-to-peak voltage of several tens volts to one hundred and several tens volts. Therefore, when the transmitter circuit is to be implemented as a silicon-based integrated circuit, a large area is required. Meanwhile, the waves reflected from the body tissues in the test object are affected by in vivo attenuation and diffusion. Thus, received signals, which are acoustic-electric converted by the individual transducers, have an extremely small amplitude. A receiver circuit that amplifies such weak signals for signal processing purposes is formed of a low-voltage device in order to deliver low noise performance, low power consumption, and small area.
Each transducer in the ultrasound probe is a transducer in which the same element performs both electric-acoustic conversion and acoustic-electric conversion. The transmitter circuit, which supplies a high voltage, and the receiver circuit, which receives a weak signal, are both connected to the same element. When the transmitter circuit supplies a high-voltage drive signal to a transducer, a switch is normally inserted between the transducer and the receiver circuit in order to electrically protect the receiver circuit formed of a low-voltage device. The switch is called a transmit receive switch.
At the time of transmission, the transmit receive switch is placed in a switch-off state to electrically protect the receiver circuit by separating it from the high-voltage drive signal generated by the transmitter circuit. At the time of reception, the transmit receive switch is placed in a switch-on state to allow a weak signal received from a transducer to pass with low loss. As the transmit receive switch plays the above role, it is necessary that the transmit receive switch have such electrical characteristics as to withstand a high-voltage signal and be formed of a high-voltage device.
Technologies concerning the transmit receive switch are described, for example, in Japanese Unexamined Patent Application Publications No. 2004-363997 (Patent Literature 1) and No. 2004-274721 (Patent Literature 2). A switch circuit described in Patent Literature 1 is configured so that a capacitor is connected between the gate and source of two NMOSFETs, which are basic elements, and adapted to retain a gate-source voltage in order to maintain a switch-on state and a switch-off state. A switch circuit described in Patent Literature 2 is configured so that the gate potentials of two NMOSFETs, which are basic elements, are increased by a PMOSFET to switch on, and that the NMOSFETs short-circuit the gate and the source to switch off.